Evidence for a proximal histidine interaction in the structure of cytochromes c' in solution: A resonance Raman study

Soret-excited resonance Raman (RR) spectra of oxidized and reduced cytochromes c' from Rhodospirillum molischianum and Rhodobacter sphaeroides, in solution, are reported. The spectra of the type I ferricytochromes c' in both species contain different extents of two forms. One of these is readily assignable to a ''normal'' five-coordinated high-spin heme. The second species with v(3) and v(10) modes at 1502 and 1635 cm(-1), respectively, is attributed to a five-coordinated intermediate-spin heme. The RR data show that the equilibrium between these two forms is species-dependent at neutral pH and 20 degrees C. The v(Fe-His) mode of the a form of reduced cytochromes c' is assigned to a band at 228-231 cm(-1), indicating that the proximal His has a strong electronegative character. X-ray crystallographic data on R. molischianum ferricyt c' show that the proximal His has no interaction with either the protein or water molecules [Finzel, B. C., Weber, P. C., Hardman, K. D., & Salemme, F. R. (1985) J. Mol. Biol. 186, 627-643]. Considering that the absence of H bonding at the coordinated histidine corresponds to a low frequency for the v(Fe-His) mode (195-205 cm(-1)), the structure and/or environment of the proximal histidine appears different for cyt c'(III) in the crystal and cyt c'(II) in aqueous solution. To account for the elevated frequency of the v(Fe-His) mode of cyt c'(II), several possibilities have been examined. Among these, we propose that a conserved Lys residue, located in the protein sequence three residues before the His ligand, can form an electrostatic interaction with the (His)N-1 atom, directly or through a water molecule. It is further suggested that this electrostatic interaction could also play a role in the high-spin <-> intermediate-spin equilibrium of oxidized cytochromes c'.